1. Field of the Invention
The present invention relates to a composition suitable for producing open-cell polyolefin resin foam. In another aspect, the present invention relates to a process for the single step production of open-cell cross-linked polyolefin resin foam from such composition.
2. Description of the Prior Art
Foams of ethylene polymer are used in large quantities as heat insulating materials, packaging materials, and cushioning materials. They are mostly of the closed-cell type which are not suitable for applications where water absorptivity and breathability are required. Breathable, open-cell foams which are available in limited quantities are coarse in cell size, poor in hand, and are used for limited applications.
The most popular open-cell foams derived from rubber or polyurethane degrade easily when exposed to ultraviolet rays and ozone.
According to the known conventional process for the production of open-cell polyethylene foam, a closed-cell foam is first prepared and subsequently the cells are broken by expansion with heating. This process gives rise to a foam which lacks resiliency and causes "bottom-out". In addition, uniform cell opening is difficult to achieve and the resulting cells are coarse in size. To remedy this drawback, a process was developed by which closed cells are opened by pressing. Unfortunately, the process disadvantages are that two steps are required, the resulting foam is thin, and the broken cell walls remain unremoved, making the foam poor in water absorption and breathability.
To overcome these shortcomings, there was proposed an improved process in Japanese Pat. No. 47-31695, which comprises the steps of cooling a closed-cell foam to a temperature under or in the neighborhood of the glass transition point and subsequently compressing the foam to rupture its cell membranes. This process needs expensive equipment and coolant such as liquified nitrogen is required.
Japanese Pat. No. 55-42100 discloses another process for preparing an open-cell polyethylene foam in one step by pressing and heating a mixture of polyethylene, blowing agent, crosslinking agent, and a large quantity of amorphous polypropylene. According to this disclosure, the amorphous polypropylene should preferably be incorporated in an amount from 30 to 50 wt.%, and that when the quantity is less than 20 wt.%, the ratio of open cells decrease. The use of amorphous polypropylene is of industrial significance because it is a by-product in the production of crystalline polypropylene and was previously discarded as a material of no commercial value. Amorphous polypropylene is a viscous claylike material at room temperature which is difficult to mold when used alone. It has been used as a bulking filler added in small quantities to low-priced foams. In such use, the incorporation of large quantities as mentioned above will adversely affect the mechanical properties and hand of the resulting foams.
The disclosure of Japanese Patent Laid-Open No. 54-63172 is characterized in that 100 parts by weight of polyolefin resin is compounded with 30 to 300 parts by weight of inorganic powder and the composition is allowed to expand to make closed-cell foam, and subsequently the closed cells are ruptured by deformation. This patent discloses that the foam is made to open more easily if the inorganic filler undergoes surface treatment which decreases the bond strength between the inorganic material and polymer. This means that it is necessary to incorporate a large amount of inorganic substance in order to accomplish the rupture of the foam cells. It can be easily conjectured that the presence of an inorganic substance will make the foam different from the original polyethylene foam in mechanical properties and hand feel. This indicates how difficult it is to make open-cell foams from a polyolefin.
After extensive studies to solve the above-mentioned problems involved in the prior art technology, we developed a process for producing open-cell foam of polyolefin which is superior in compressive property, water absorption, weatherability, and hand and which keeps the characteristics of polyolefin even when a large quantity of additive is incorporated, by a simple process without additional process for cell rupture. The elements which constitute the technology of the present invention are summarized as follows:
First, the relationship between the decomposition temperature of the blowing agent and the decomposition temperature of the crosslinking agent is inverted from that in the conventional technology. Secondly, the cells before and after rupture are made fine and uniform in size by adding a small quantity of a trifunctional compound and silicone oil or a derivative thereof. Thirdly, the foam can be produced batchwise in a furnace as well as continuously. Finally, the composition before foaming can be in the form of pellets, powder, film, sheet, and other shapes of moldings.
It is, therefore, an object of this invention, therefore, to provide an open-cell foam which is outstanding in breathability, water absorption, weather resistance, flexibility, and hand which are attributable to the open-cell structure.
It is another object of the invention to provide a process which permits the production of open-cell foam by a single heating step, without the need for a crushing step to open cells.